Emergence of charge density wave domain walls above the superconducting dome in 1T-TiSe2

When the charge density wave state in TiSe 2 is suppressed by hydrostatic pressure or chemical doping, superconductivity appears. This suggests the presence of a quantum critical point. Yet a high pressure X-ray study unexpectedly finds that the quantum critical point is nowhere near the superconducting dome. Superconductivity in so-called unconventional superconductors is nearly always found in the vicinity of another ordered state, such as antiferromagnetism, charge density wave (CDW), or stripe order. This suggests a fundamental connection between superconductivity and fluctuations in some other order parameter. To better understand this connection, we used high-pressure X-ray scattering to directly study the CDW order in the layered dichalcogenide TiSe 2 , which was previously shown to exhibit superconductivity when the CDW is suppressed by pressure 1 or intercalation of Cu atoms 2 . We succeeded in suppressing the CDW fully to zero temperature, establishing for the first time the existence of a quantum critical point (QCP) at P c = 5.1 ± 0.2 GPa, which is more than 1 GPa beyond the end of the superconducting region. Unexpectedly, at P = 3 GPa we observed a reentrant, weakly first order, incommensurate phase, indicating the presence of a Lifshitz tricritical point somewhere above the superconducting dome. Our study suggests that superconductivity in TiSe 2 may not be connected to the QCP itself, but to the formation of CDW domain walls.